1. Field of the Invention
The present invention relates to a process for producing ferrous powder metal parts, and particularly relates to a process for producing ferrous powder metal parts having high surface hardness and superior rolling contact fatigue properties.
2. Description of the Invention Background
Many useful mechanical properties of powder metal ("P/M") parts improve with increases in part density. This relationship is particularly valid for dynamic properties of parts such as impact fatigue and rolling contact fatigue, which may increase dramatically as part density approaches theoretical density. Increasing the density of a P/M part reduces the prevalence of pores. Fatigue cracks typically originate at the sharp edges of pore sites and failure of the entire part may result from these cracks.
The increased fatigue properties resulting from increased density are of prime importance in the operation of structural components which undergo high cyclic stresses during operation. Components such as gears, cams and sprockets are in continuous high-stress rolling contact and may crack, break, or splinter after continuous contact under heavy load. P/M parts subjected to the cyclic stress of repeated rolling contact must also have high surface hardness. However, P/M parts with high densities typically do not have high surface hardness absent additional treatment procedures.
There are a number of known techniques for producing high density ferrous P/M parts. One such technique is simply the use of purer iron powders in a conventional mold and sinter cycle. Purer iron powders result in enhanced part density because interstitial impurities, particularly carbon but also oxygen and nitrogen, reduce the compressibility of iron powders. Current technology yields maximum green densities of 7.2-7.4 g/cc (92-94% of theoretical maximum density) by molding pure iron powder at 60 tsi (tons per square inch). Green density refers to the density of a green compact, i.e., a compacted powder mass, which has not been sintered. The green compact is then sintered at about 2050.degree.-2300.degree. F., typically in a vacuum furnace or in the inert gas environment of the heating zone of a belt or pusher furnace incorporating silicon carbide heating elements. Weight loss and expansion during sintering typically reduces the maximum final density of sintered compacts produced by from pure iron powder by this "mold-sinter" process to between about 6.8-7.3 g/cc.
Lower maximum final densities are achieved if iron alloy powders are used in a mold-sinter process. However, recently available molybdenum-iron alloy powders can be processed to approximately 7.25 g/cc density by a mold and sinter cycle. Parts produced by the mold-sinter process generally have low strength and surface hardness and must be subjected to additional procedures to improve structural properties such as strength, hardness and toughness.
A second technique to produce high density ferrous P/M parts is the double press/double sinter process. In that process, a mix of iron powder and/or iron alloy powder is blended with loose graphite powder, and possibly with other alloy additives, and is molded at room temperature at about 30-50 tsi. The green compact ms then pre-sintered at approximately 1600.degree. F. to provide a pre-sintered compact having a density of 6.8-7.2 g/cc. The pre-sinter temperature is selected to minimize the solution of carbon in the ferrous compact. Because the pre-sintering temperature allows minimal solution of carbon in the iron, the pre-sinter anneals and softens the iron so it can be further worked. If the pre-sinter temperature is too high, solution of carbon in the iron will make it much stronger and very resistant to subsequent working. After pre-sintering, the compact is placed back into the die and is re-pressed at room temperature and about 50-60 tsi, resulting in a re-pressed compact with a density of about 7.2-7.5 g/cc. Finally, the repressed compact is sintered a second time at about 2050.degree.-2300.degree. F.
The double press/double sinter process typically produces parts in the 7.3-7.5 g/cc range having properties significantly improved relative to parts produced by a mold-sinter process. The repressed parts may then be heat treated to improve structural properties.
The hot forming process is a third technique for producing high density ferrous P/M parts. In the hot forming process, the starting powder is first molded and sintered, usually at around 40 tsi and 2050.degree. F. The part is then reheated to forging temperatures in the 1500-1800.degree. F. range, placed in a heated die, and hot formed at 50-60 tsi to a high density. Again, for optimum properties, the part is heat treated in additional operation. Densities by the hot forming process can approach theoretical maximum density, 7.80-7.85 g/cc. These density values may be compared with iron's 7.87 g/cc theoretical maximum density.
Each of the above processes has disadvantages. The mold-sinter process does not produce densities sufficient for structural parts subjected to high stresses. In addition, in the absence of carbon, parts produced by a pure iron powder mold-sinter technique have relatively low strength and surface hardness and are, therefore, unsuited for applications where they are subjected to high cyclic stresses.
Relative to a mold-sinter technique, the double press/double sinter process provides a higher density part, but the process is quite expensive because it requires two dies, two pressings and two furnace operations. The cost of dies is a major portion of the cost in manufacturing P/M parts. Any additional heat treatment employed after the second sinter step necessarily adds the expense of a third furnace operation.
The hot forming process provides the highest currently attainable densities for ferrous P/M parts. However, size control is difficult during the hot forming process. Size control is particularly important in the manufacture of structural parts, which may require exacting tolerances. The hot forming process is also relatively expensive because it includes two furnace operations and the use of a heated die during the hot forming operation. Typically one to ten million tools may be produced from a single pressing die in P/M processing. However, the heated forming dies used in the hot forming process must be replaced after only about 50,000 hot forming cycles, substantially increasing the cost of the finished part.
As discussed above, P/M parts produced by the above three techniques are normally subjected to separate final heat treatments to maximize the parts' structural properties. In a typical heat treatment, a finished part is heated to 1400.degree.-1600.degree. F. and then oil quenched. The resulting hard and brittle part is tempered at 300.degree.-600.degree. F. to impart toughness without significant reduction in part hardness or strength. In both the double press/double sinter and hot forming processes 0.3-1.0 weight percent carbon powder is added to the initial powder mix to increase the strength and surface hardness of the finished part and to optimize the results of the subsequent heat treatments. The carbon content is slightly diminished to between 0.3-0.8 weight percent after carbon loss from furnace operations.
Although carbon powder must be added to optimize structural properties produced by heat treatment, the free carbon in the pre-compacted powder reduces the compressibility of the powder and, therefore, reduces the both the maximum density and the fatigue resistance of the finished part. Therefore, although it is known to use carbon powder-free materials in P/M processing to maximize density, methods for producing structural parts wherein the initial powder mix is devoid of powdered carbon are not widely practiced because optimum strength, hardness and fatigue properties may not be achieved.
U.S. Pat. No. 2,489,839, entitled "Process for Carburizing Compacted Iron Articles" and referred to herein as the '839 patent, does provides a method for producing ferrous P/M parts from an initial powder substantially free of carbon powder. The '839 patent discloses a multiple sinter/repress process for producing ferrous P/M parts having a substantially uniform carbon content. The '839 process' starting material is preferably a pure electrolytic iron powder which may contain desired amounts of powdered metal alloying ingredients and which is substantially free of carbon. The initial powder mix of the '839 patent is compacted at less than 40 tsi, sintered and then repressed at or above 60 tsi. To provide the compact with a uniform carbon content, the repressed compact is preferably treated in a two-step process consisting of (i) an initial carburizing step at 1600.degree.-2000.degree. F. to produce a high carbon outer layer and (ii) a carbon homogenization step wherein the carbon in the high-carbon layer is redistributed throughout the compact.
The homogenization step of the '839 patent is carried out by heating the compact for an extended period in a controlled environment having a carbon concentration chosen to provide a desired carbon concentration throughout the steel compact. The homogenization step is intended to uniformly distribute the carbon throughout the iron part, rather than concentrate the carbon near the surface of the part. Migration of the carbon from the outer layer to the interior of the compact during the homogenization step necessarily reduces the carbon level in the surface of the part which in turn lowers hardness and rolling contact fatigue properties.
Considering the disadvantages of the above processes, an objective of the present invention is to provide a process for manufacturing P/M parts having high density, high surface hardness and superior rolling contact fatigue properties.
A further object of the invention is to produce a high density P/M part having high surface hardness and superior rolling contact fatigue properties with a minimum number of pressing, sintering and heat treating operations.